1. Field of the Invention
The present invention relates to a high-brightness compound semiconductor light-emitting device, and more particularly to an epitaxial wafer for a light-emitting diode having a double hetero-structure with an AlGaInP active layer, and to a light-emitting diode that uses the epitaxial wafer.
2. Description of the Prior Art
Light-emitting diodes (LEDs) utilizing epitaxially grown layers formed on a compound semiconductor substrate are featured by low power consumption, long life, high emission efficiency and high reliability, and are widely used as a light source in various display devices. AlGaInP is a direct-transition type semiconductor crystal having the highest bandgap energy among group III-V mixed crystals excluding nitrogen compounds. Because LEDs using AlGaInP can emit light at a shorter wavelength than conventional AlGaAs-based LEDs fabricated by the liquid-phase epitaxy (LPE) method, increasingly it is being used for high-brightness light-emitting element applications. In particular, interest has recently focussed on LEDs that have a GaAs substrate and a double hetero-structure (DH) with an AlGaInP active layer that is lattice-matched with the substrate, since such LEDs are able to emit light at high brightness from green to red. Moreover, owing to the fact that such AlGaInP mixed crystal LEDs have less Al in their composition, they are also regarded as having superior humidity resistance compared to AlGaAs-based LEDs fabricated by the LPE method.
A drawback of AlGaInP-based LEDs is that since they have a thin-film structure formed by the metal-organic chemical vapor deposition (MOCVD) method, current injected from the electrodes is not readily diffused. Hence, emission efficiency is not always sufficient, especially in the short wavelength regions. This has led to various attempts at structural improvements. In one example, JP-B-6-103759 discloses an LED structure provided with a current diffusion layer that is a thick layer of AlGaAs. As illustrated by FIG. 1, this LED comprises an n-type GaAs substrate 1, an n-type AlGaInP lower cladding layer 2 having a thickness of 1 .mu.m, an AlGaInP active layer 3 having a thickness of 0.5 .mu.m, a p-type AlGaInP upper cladding layer 4 having a thickness of 0.2 .mu.m, a p-type AlGaAs current diffusion layer 5 having a thickness of 3 .mu.m, a p-electrode 6 and an n-electrode 7.
A condition required of the current diffusion layer of the LED is that it be transparent to the light from the light-emitting portion. For an AlGaAs layer satisfying that condition to be used as the current diffusion layer, the Al content of the layer has to be higher than a certain value to ensure that the bandgap energy of the AlGaAs current diffusion layer is greater than the bandgap energy of the active layer. However, the higher the Al content, the more prone the AlGaAs is to oxidation. The mixed crystal Al ratio of AlGaAs that is transparent to light in the visible spectrum emitted by an AlGaInP-based LED is in the order of 0.7, around the same as in the composition of cladding layers of an AlGaAs LED fabricated by the LPE method. Therefore, while an AlGaInP LED may emit light at a higher brightness than an AlGaAs LED fabricated by the LPE method and also be superior in humidity resistance, resistance to humidity is still a problem in a structure in which the current diffusion layer is formed of AlGaAs.
In view of the purpose of current diffusion, the thicker the AlGaAs layer, the better. JP-A-4-212479 proposes a thickness of 5 to 30 .mu.m, while U.S. Pat. No. 5,008,718 proposes a thickness of 2 to 30 .mu.m, more preferably 5 to 15 .mu.m (see column 3, line 16, onward). There is also an example of an AlGaAs current diffusion layer 7 .mu.m thick being used (see Appl. Phys. Lett. Vol. 58 (10), Mar. 11, 1991). Thus, in an LED that uses an AlGaAs current diffusion layer, in order to enhance the current diffusion effect the thickness of the layer is increased, but doing this also increases the Al content, thereby reducing the humidity resistance property by making the AlGaAs more prone to oxidation.
Furthermore, even when provided with such a current diffusion layer to enhance the current diffusion effect, conventional LEDs do not provide sufficient brightness for use outdoors and the like where high brightness is required. Here, brightness is proportional to light emission intensity with the addition of a visibility effect value. Another problem is that increasing the number of the hetero-junction increases device resistance, so a higher operating voltage is required. The reason why high brightness cannot be obtained is considered to be that the double hetero-structure is not lattice matched, in addition to which good-quality epitaxial layer growth is not obtained owing to the fact that the many hetero-junction surfaces are involved. In the case of the four-element mixed crystal (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P, lattice matching with the GaAs substrate has always been regarded as being when y is 0.5. However, this is at room temperatures, but with this composition the lattices do not match at epitaxial growth temperatures. Namely, at room temperature a GaAs substrate has a lattice constant of 0.56533 .mu.m, and at an epitaxial growth temperature of say 780.degree. C. the lattice constant is 0.56804 nm, which is a change of 0.48%. While (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P used for the cladding layer has a lattice constant of 0.56640 nm at room temperature, at 780.degree. C. the lattice constant is 0.56837 nm, an increase of 0.348%. Thus, the degree of lattice mismatch between the substrate and the cladding layer is 0.19% at room temperature and 0.058% at 780.degree. C. So, while on first impression epitaxially grown layers of (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P might appear to be lattice-matched, there is quite a degree of mismatch.
The present inventors conducted detailed studies and found that even if lattice matching is achieved at room temperature, it is all the more important to effect lattice matching at the epitaxial growth temperature. Good crystal growth resulting in high brightness is obtained if there is lattice matching during the epitaxial growth process, since even if defects are produced in the crystal lattice after cooling to room temperature, such defects are not of a magnitude enough to reduce the brightness to any real extent.
If good-quality epitaxially grown layers are obtained, current diffusion in the upper cladding layer is also improved, to the extent that even without the provision of a special, thick current diffusion layer, the device resistance can be reduced, which also makes it possible to use a lower operating voltage (the V.sub.F characteristic). Moreover, not having to increase the thickness of the epitaxially grown layers means that the Al concentration is not increased, which resolves the problem of device degradation and therefore results in high device reliability.
An object of the present invention is to provide a high-brightness, long-lasting AlGaInP light-emitting diode having an AlGaAs current diffusion layer that possesses a sufficient current diffusion effect while also maintaining fully adequate humidity resistance, and a wafer used to form the light-emitting diode.